Central solar receiver

ABSTRACT

A central solar receiver comprising a housing with a window for the admission of incident concentrated solar radiation, a volumetric solar absorber within the housing comprising a base body holding an array of absorber members spaced from each other and projecting from one face thereof with their free ends facing said window, means for the injection of working fluid into the volumetric solar absorber in flow directions which intersect the absorber members and means for the withdrawal of heated working fluid. The window of the central solar receiver is a tubular frusto-conical body coaxial with the housing, whereby the receiver is adapted for operation at elevated pressure.

GLOSSARY

Some of the terms that will be used in the following description andclaims have the following meaning:

Solar absorber--a body or construct capable of absorbing and convertinginto heat impinging solar radiation.

Working fluid--a fluid flowing in contact with a solar absorber andtaking up heat therefrom by indirect or direct heat exchange. Theworking fluid may serve as heat carrier for removal of the heatgenerated in the solar absorber. In addition or alternatively it mayconsist of a mixture of two or more components which, upon contact withthe hot solar absorber, are induced to react with each other, i.e. toperform a so-called thermochemical process.

Central solar receiver--a device with a chamber comprising a solarabsorber and means for the circulation of a working fluid in indirect ordirect heat exchange relationship therewith; so called because inoperation it is mounted in the center, i.e. the focal region of a solarradiation concentrator.

Volumetric solar absorber--a solar absorber fabricated in form of athree-dimensional matrix enabling the throughflow of a working fluid indirect heat exchange relationship therewith.

Receiver's window--a transparent section of the receiver's wall, whichallows highly concentrated sunlight to enter the receiver's chamber,while preventing the pressurized working fluid from escaping.

FIELD AND BACKGROUND OF THE INVENTION

The invention relates to central solar receivers with volumetric solarabsorber.

A central solar receiver absorbs concentrated sunlight at a hightemperature, commonly about 700°-1300° C. and transfers the heat fromthe solar absorber therein to a working fluid which either serves asheat carrier fluid or else is designed to perform a heat induced,possibly catalyzed, endothermic chemical reaction between components ofthe working fluid. For various industrial applications such as theoperation of gas turbines for electricity generation or the performanceof endothermic reactions of the kind specified, it is necessary that theworking fluid circulates through the system at an elevated pressure ofsay 10-30 atmospheres. At such a pressure the density of the circulatinggaseous working fluid is much higher than in the unpressurized state andconsequently pressure losses during circulation are lower.

A directly irradiated central solar receiver with volumetric absorbercomprises a housing having a window for the admission of incidentconcentrated solar radiation and one of the most critical problemsassociated with pressurized central solar receivers is the mechanicalstrength of the receiver's window. Materials that have the requiredoptical and thermal properties tend to be brittle, which means thatwhile they can withstand large compression stresses they tend to crackor shatter under even relatively small tension stresses. Stresses in thewindow are created by the gas pressure in the interior of the receiverand also by uneven thermal expansion of the window and other receivercomponents as they heat up during operation. Non-uniform stresses exist,as a rule, at different points in any window of a central solar receiverand consequently hitherto central solar receivers could not be operatedat elevated pressures.

In IL 97091 and the corresponding U.S. Ser. No. 07/820,418, U.S. Pat.No. 5,245,986 issued Sep. 23, 1993, EP 495395 A1 and patent applicationsin other jurisdictions, there is described an improved central solarreceiver comprising a housing with a window for the admission ofincident concentrated solar radiation accommodating a volumetric solarabsorber having a base body with an array of absorber members spacedfrom each other and projecting from one face thereof with their freeends facing the window, the working fluid being injected into thevolumetric solar absorber in a flow which intersects the absorbermembers.

It is the object of the present invention to provide a central solarreceiver based on the teachings of our IL 97091 and designed foroperation at elevated pressure.

It is a further object of the present invention to provide a window unitadapted for incorporation in the housing of a central solar receiveroperating at elevated pressure.

It is yet another object of the present invention to provide avolumetric solar absorber for incorporation in a central solar receiverof the kind specified operating at elevated pressure.

SUMMARY OF THE INVENTION

In accordance with the invention there is provided a central solarreceiver comprising a tubular housing with a window for the admission ofincident concentrated radiation, a volumetric solar absorber within thehousing comprising a base body holding an array of absorber membersspaced from each other and projecting from one face thereof with theirfree ends facing said window, means for the injection of working fluidinto the volumetric solar absorber in flow directions which intersectthe absorber members and means for the withdrawal of heated workingfluid, characterized by said tubular volumetric solar being anaxi-symmetric tubular body in which said absorber members projecttowards the axis, which tubular volumetric absorber is coaxial with saidtubular housing and has a radiation admitting aperture; and by saidwindow being an axi-symmetric tubular frusto-conical body coaxial withand surrounded by said volumetric solar absorber; whereby the centralsolar receiver is adapted for operation under elevated pressure.

Due to the frusto-conical shape of the window any pressure actingthereon from within the housing of the central solar receiver accordingto the invention produces compression stress components which the windowcan intrinsically withstand.

Preferably, the frusto-conical window comprises at both ends cylindricalrim portions by which it can be set and held.

If desired, the frusto-conical window in a central solar receiveraccording to the invention may be of the double pane type.

In a central solar receiver according to the invention, the tubularfrusto-conical window has large-diameter and small-diameter endportions. Of these, the former forms the radiation admitting apertureand is located within a matching opening in one of the end walls of thehousing and the latter is thus rear of the window. Preferably the rearof the window is connected to the receiver's housing by means of abellows which absorbs any movements of the window relative to thehousing without creating additional steam on the window.

In accordance with one embodiment of the invention, reflector means areprovided near the inner small diameter end portion of the tubularfrusto-conical window adapted to reflect into the solar receiver anyincident concentrated solar radiation that has not penetrated throughthe window.

The axi-symmetric tubular volumetric solar absorber in a central solarreceiver according to the invention may be of any suitable shape such aspolyhedral-prismatic, cylindrical, frusto-conical, paraboidal,ellipsoidal, etc.

In a preferred embodiment of a central solar receiver according to theinvention, means are provided for sweeping the inner face of said windowwith pressurized working fluid whereby the window is cooled.

It is further preferred to provide means for sweeping the outer face ofsaid window with ambient air whereby the window is cleaned and furthercooled.

The invention further provides for use in a central solar receiver ofthe kind specified, an axi-symmetrical tubular volumetric absorber bodyfitted with absorber members projecting from the inner face thereof.

Still further, the invention further provides for incorporation into thebody of a central solar receiver, an axi-symmetric tubularfrusto-conical transparent body suitable for forming a window thereof.Preferably the said tubular frusto-conical transparent body comprises atboth ends cylindrical rim portions. The window in a central solarabsorber according to the invention is made of conventional materialswith the desired optical and thermal properties such as, for example,fused quartz.

DESCRIPTION OF THE DRAWINGS

For better understanding, a specific embodiment of the invention willnow be described, by way of example only, with reference to the annexeddrawings in which:

FIG. 1 is an axial section of a central solar receiver according to theinvention;

FIG. 2 is a section along line II--II of FIG. 1; and

FIG. 3 is a section along line III--III of FIG. 1.

DESCRIPTION OF A SPECIFIC EMBODIMENT

The central solar receiver according to the invention here showncomprises a metal housing 1 with front and rear end walls 2 and 3 andlined from within by an insulation layer 4. Through the central circularhole 5 of the front end wall 2 there is inserted a tubularfrusto-conical window 6 suitable for the admission of solar radiationand for withstanding high temperatures. Near its large diameter endwindow 6 comprises a cylindrical 7 and near its small diameter end thewindow comprises a cylindrical rim 8. The setting of window 6 by meansof its rim 7 in the front end wall 2 comprises a seal 9 held within asuitably shaped annular groove in end wall 2 and a positioning O-ring10.

Near the narrow end portion of window 6 there is located a metal block12, e.g. of aluminum, having a hollow portion 13 comprising acylindrical part 14 and a highly polished, reflector part 15. Thesetting of window 6 by means of its rim portion 8 in block 12 comprisesa seal 16 held within a suitably shaped annular groove in the block, anda positioning O-ring 17.

Block 12 is held within an insulation sleeve 18 and is connected to oneend of a bellows 19 whose opposite end bears on a central sealing plate20 of the rear end wall 3. A tube 21 with connector 22 and a coaxialsleeve 23 with a connector 24 serves, respectively, for the ingress ofcooling water into the interior of the hollow portion 13 of block 12 andthe egress of the cooling water therefrom whereby the reflector portion15 of block 12 is cooled.

Block 12 comprises a plurality of air passage bores 26 and sealing plate20 comprises a plurality of air passage bores 27 providing together witha cylindrical duct 28 surrounding sleeve 23 a passage for airflow fromthe inner space of window 6 to the rear of housing 1.

Inside housing 1 there is mounted a volumetric solar absorber 30comprising a frusto-conically shaped base body 31 holding an array ofspike-shaped absorber members 32 projecting from the inner surfacethereof, both the base body 31 and the absorber member 32 being made ofa suitable heat-resisting material such as ceramic material, aceramic-coated metal alloy, silicone carbide, alumina, a special typestainless-steel, a nickel alloy or the like. Where the central solarabsorber 30 serves for performing a heat induced endothermal chemicalreaction between components of the working fluid, the absorber members32 may be coated with a suitable catalyst.

Near its large diameter end portion the volumetric solar absorber 30 isfitted with a perforated metal ring 33 serving for the injection ofworking fluid from an annular duct 34 into the absorber so as tointersect the absorber members 32.

To the back of the volumetric solar absorber 30, and bearing on thesmall diameter end thereof, there is provided an annular chamber 35fitted with inlets 49 and a tubular vent 36 and surrounded by a layer ofinsulating material 37 which also extends to the outer rear portion ofthe volumetric solar absorber 30.

In a region near to the rear end wall 3, housing 1 is fitted with aplurality of pairs of axial inlet pipes 38', 38" linked to a circularmanifold (not shown) of which each pipe 38' opens into a rear,annular/cylindrical duct 39 and each pipe 38" opens into a cylindricalduct 40 which leads to the annular duct 34. Ducts 39 and 40 areseparated by an annular partition 41 whereby working fluid injected viapipes 38' and 38" is conducted separately to an annular nozzle 42directed towards the rear portion of window 6 and to the annular duct 34from which it is injected via injector 33 into the volumetric solarabsorber 30.

If desired, the annular duct 40 may be subdivided by axially orientedinsulating partitions into two or more separate ducts each in form of atubular segment and each being fitted with a separate inlet pipe.Alternatively the annular duct may be subdivided by means of a spirallyshaped partition into a number of annular ducts. In either way it ispossible to produce non-uniform fluid injection which may becomedesirable under certain thermal conditions in the receiver.

The front end wall 2 comprises a plurality of ducts 44 connected to airpumping means (not shown) and opening into an annular groove 45 wherebyair is injected into the cavity of mirror 6 and flows along the outersurface thereof to be discharged via bores 26, duct 28 and bores 27.

The operation of the central solar receiver according to the inventionis as follows:

Concentrated solar radiation is admitted via the aperture formed by thelarge diameter end of the tubular frusto-conical window 6. Some of theincident solar radiation penetrates directly through the window andimpinges on the absorber members 32 of the volumetric solar absorber 30.Nearly all the incident solar radiation that does not penetrate acrosswindow 6 impinges the reflector portion 15 of body 12 and is reflectedtherefrom across window 6 on to the solar absorber 30. The combinedincident solar radiation heats up the volumetric solar absorber 30.

Pressurized working fluid is injected into the receiver via theplurality of pairs of pipes 38', 38" from the manifold to which they areconnected. The part of the injected working fluid that arrives via pipes38" is conducted through ducts 40 and 34 and is injected to thevolumetric solar absorber 30 via injector 33 with the flow directionbeing such that the injected working fluid intersects the absorbermembers 32.

The pressurized working fluid injected via pipe 38' is ejected via theannular nozzle 42 and flows along the inner face of window 6 whereby thewindow is cooled, and then mixes with the working fluid injected via theannular injector unit 33. After having travelled across the array ofabsorber members 32 the working fluid which, as the case may be, iseither simply heated up or else as the product of an endothermalchemical reaction, enters the annular chamber 35 via the ports 49 and isdischarged via vent 36 either as a reaction product or as hot gas forindustrial use, e.g. for operating electric power generating turbines.

It should be noted that the geometry of the injection and discharge ofthe pressurized working fluid can be modified to meet specific designrequirements.

In the course of operation the reflector 15 is cooled by cooling wateringressing via pipe 21 and egressing via sleeve 23.

In the course of operation air is injected continuously via ducts 44sweeping the outer surface of window 6 whereby it is cooled and any dustdeposited thereon is removed. The sweeping air is discharged via bores26, duct 28 and bores 27.

Thus, the window is cooled from the inside by the working gas injectedthrough nozzle 42, and from the outside by the air from duct 44.

Due to the frusto-conical shape of the tubular window 6, the pressureacting thereon from within has components which are parallel to thesurface of the window and which accordingly result in a compression.Accordingly, this window is well suited to withstand the elevatedpressure prevailing within the central solar receiver according to theinvention in consequence of the injection of pressurized working fluidthrough inlets 38' and 38".

In the course of operation any thermal expansion whether symmetric orasymmetric, is absorbed by bellows 19 and as a result, any cracking ofthe window or other components of the receiver is avoided.

It will be readily understood by those skilled in the art that thefrusto-conical shape of the volumetric solar absorber 30 is not criticaland that instead, any other axi-symmetric configuration may be used suchas, for example cylindrical, paraboidal, ellipsoidal and the like. Itwill be further appreciated that the spike-like configuration of theabsorber members 32 shown herein is not critical and any other suitableconfiguration may also be used such as, for example rods, hollowcylindrical tubes, frustum, flat panels and the like.

An experimental model of the receiver herein described was tested at apower level of about 10 kW. The receiver pressure during the tests wasbetween 15 and 25 atmospheres; the maximum absorber temperature wasabout 1100° C.; the total exposure time was about 50 hours. None of thereceiver's components was damaged or failed during the tests.

We claim:
 1. A central solar receiver comprising:a tubular housinghaving an opening and a window for the admission of incidentconcentrated solar radiation; a volumetric solar absorber within thehousing and comprising a base body, an array of absorber members held bythe base body and spaced from each other, the absorber membersprojecting from one face of said volumetric solar absorber with freeends of the absorber members facing said window; injection means forinjecting a working fluid into the volumetric solar absorber in flowdirections intersecting the absorber members and withdrawing means forwithdrawing heated working fluid, wherein: said volumetric solarabsorber is formed as an axi-symmetric, frusto-conical, tubular bodycoaxial with said tubular housing and having a radiation admittingaperture underlying said opening of the housing, said absorber membersprojecting towards a longitudinal axis of said volumetric absorber; saidwindow is formed as an axi-symmetric, frusto-conical, tubular bodycoaxial with and surrounded by said volumetric solar absorber; and arear of said window is connected to said housing by a bellows, thebellows absorbing thermal expansion of said window; wherein the centralsolar receiver is adapted to operate under an elevated pressure of theworking fluid.
 2. The central solar receiver according to claim 1,wherein said window has a double pane.
 3. The central solar receiveraccording to claim 1, further comprising means for sweeping the innerface of said window with pressurized working fluid from inside thereceiver to cool the window.
 4. The central solar receiver according toclaim 1, further comprising means for sweeping the outer face of saidwindow with ambient air, wherein the window is cleaned and cooled.